1. Field of the Invention
The present invention relates to a vibration damping device for a vehicle body that damps vibrations of a vehicle body and improves the riding comfort of the vehicle.
2. Description of the Related Art
The body of a vehicle is known to be slightly elastically deformed by an external force applied at the time of traveling and, because of the elastic deformation, generate a vibration having a small amplitude. The amplitude of the vibration is several ten μm to about 1 to 2 mm. The external force that causes the elastic deformation is applied from the engine or the wheels running on an uneven road surface to the vehicle body.
Conventionally, to damp a vehicle body vibration of this type, a vibration damping device is mounted in a vehicle body in some cases. As a conventional vibration damping device for a vehicle body, for example, Japanese Patent No. 4627389 discloses a hydraulic device.
The hydraulic vibration damping device for a vehicle body disclosed in Japanese Patent No. 4627389 includes a hydraulic cylinder portion configured to generate a damping force and a connecting rod portion coupled with a piston rod projecting from the hydraulic cylinder portion. The hydraulic cylinder portion is mounted at, for example, one end of the vehicle body in the side-to-side direction. The distal end of the connecting rod portion is mounted on, for example, the other end of the vehicle body in the side-to-side direction.
The hydraulic cylinder portion includes a hydraulic cylinder, a piston and a free piston that are movably fitted in the hydraulic cylinder, and a piston rod connected to the piston.
The piston divides a hydraulic oil chamber in the hydraulic cylinder into two oil chambers. The piston is provided with a hydraulic oil passage that allows the two oil chambers to communicate with each other, and a check valve to open/close the hydraulic oil passage and generate a damping force. The piston rod extends through one oil chamber and projects outside the hydraulic cylinder.
The free piston divides the inside of the hydraulic cylinder into a hydraulic oil chamber and a gas chamber filled with a high pressure gas. The hydraulic oil in the hydraulic oil chamber is always pressurized by the high pressure gas.
The hydraulic cylinder portion itself is directly fixed to one end of the vehicle body in the side-to-side direction or the back-and-forth direction. On the other hand, the piston is fixed to the other end of the vehicle body in the side-to-side direction or the back-and-forth direction via the piston rod and the connecting rod.
In the conventional vibration damping device for a vehicle body, the piston and the hydraulic cylinder relatively move due to the vibration of the vehicle body, and the hydraulic oil passes through the check valve of the piston to generate a damping force.
In this vibration damping device for a vehicle body, to change the magnitude of the generated damping force, the check valve needs to be exchanged.
As a conventional hydraulic buffer capable of changing the damping force, for example, Japanese Patent Laid-Open No. 10-331898 describes a shock absorber.
The shock absorber disclosed in Japanese Patent Laid-Open No. 10-331898 includes a cylinder, a piston rod inserted into one end of the cylinder, and a first piston and a second piston that are mounted on the piston rod. The first piston is fixed to the piston rod on the other end side of the cylinder. The second piston is fixed on the piston rod on one end side of the cylinder with respect to the first piston. The first piston and the second piston divide the inside of the cylinder into three oil chambers. The three oil chambers include a first oil chamber between the first piston and the other end of the cylinder, a second oil chamber between the two pistons, and a third oil chamber between the second piston and the one end of the cylinder.
Each of the two pistons is provided with a leaf valve that generates a damping force. The leaf valve is provided in the hydraulic oil passage extending through the piston. The leaf valve closes when the pressure difference between the two oil chambers divided by the piston is equal to or smaller than a predetermined threshold, and opens when the pressure difference exceeds the threshold.
A hydraulic oil passage that allows the second oil chamber and the third oil chamber to communicate with each other is provided in the piston rod. A needle valve that defines an adjustable valve is provided along the axis of the piston rod. The needle valve changes the sectional area of the hydraulic oil passage due to a tapered portion fitted in the hydraulic oil passage in the piston rod. The needle value is integral with an operation rod extending up to the projecting end of the piston rod. The operation rod threadably engages with the piston rod.
In this shock absorber, when the operation rod is rotated with respect to the piston rod to screw or loosen the threadably engaging portion, the aperture ratio of the needle valve continuously changes. This makes it possible to arbitrarily adjust the magnitude of the generated damping force in an onboard state.